When it comes to reliability and longevity of HVAC equipment, run time can play a major role. I'm sure we've all heard the term "short-cycling" before, it's basically when equipment starts and stops in a short period of time. Let's look into "short period of time", in the HVAC world.

We have in our world, a term called "Steady State", the best way to describe this is when things start to settle -out or hit cruise control if you will.

In the combustion process, its when the chemicals in the reaction of combustion reach their steady levels for this process. During the reaction (light-off) these numbers are kind of all over the place and start moving rapidly. We have drastic temperature changes during the process that also lend its hand to the movement of these numbers. The blower or pump is also starting and ramping up to speed. It generally takes about 10-15 minutes for a gas fired appliance to reach steady state. If we don't allow an appliance to reach steady state, there is an associated risk of damaging the equipment. The rapid heating up and cooling down of metal can shorten its lifespan. There is also the chance of condensing in area's where we don't want condensation to take place (which is high in acid), causing damage. It is virtually impossible to achieve a true steady state, as room temperatures start to change, but we can get pretty close.

In refrigerant cycles, air conditioning or heat pump, steady state is just as important. The compressor operates best when it can run at its desired compression ratio, and temperature, which impacts electrical consumption. The refrigerant goes through state changes (temperature changes) as it runs through the system. If steady state cannot be reached, the compressor cannot work properly and the process becomes less than efficient. This is why equipment manufactures will state in their manuals to allow the system to run a minimum of 15 minutes before making any charge adjustments, and another 15 minutes after an adjustment has been made. The ensures the refrigerant goes through state changes where it should, and the system returns to steady state.

If the equipment is sized, installed, and commissioned correctly, it should run approximately 3-5 cycles per hour (10-20 minutes per cycle). High efficiency equipment will tend to be on the lower end due to its total combustion process, and rated temperature design taking longer to reach. These times and cycles can vary depending on changing load conditions. So we do our best.

Component and equipment failure is almost a promise as we shorten the lifespan by increasing the amount of cycles.

Comfort can also be impacted by equipment not being able to reach steady state or by short cycling. We lose the ability to humidify or de-humidify the space properly, It can create a "breezy" feeling, There can be hot-spots and cold spots created in a space, and improper fresh air exchange is also sacrificed.  

Tips:

•  Have a proper load calculation and design performed
• Select the correct temperature control location
• Add or relocate temperature controls (sensors)
• Perform start up sheets or commissioning reports

So in summary:

Main Takeaways:

• Short Cycling Harms Equipment – Frequent starts and stops reduce reliability and lifespan.
• Steady State is Crucial – HVAC systems operate best once temperatures and pressures stabilize (10-15 min for gas appliances, 15 min for refrigerant cycles).
• Impact on Efficiency – Running at steady state improves energy efficiency and performance.
• Manufacturer Guidelines Matter – HVAC systems should run at least 15 minutes before charge adjustments to allow proper refrigerant state changes.
• Recommended Cycle Rate – Ideally 3-5 cycles per hour (10-20 min per cycle), with high-efficiency systems tending toward the lower end.
• Short Cycling Increases Wear – More cycles lead to faster component failure and higher maintenance costs.
• Comfort & Air Quality Affected – Short cycles reduce dehumidification, create hot/cold spots, and impact fresh air exchange.
• Best Practices:
  • Perform proper load calculations and design.
  • Ensure correct thermostat placement.
  • Use temperature sensors if needed.
  • Follow startup and commissioning guidelines.

Carlos